This invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus having a color-correcting processing.
There has been known an image forming apparatus in which an original color image on an image input medium such as a color photographic film, a television, a copy sheet or the like is read by an image reader to obtain an original image data, the original image data is separated into three color components (red, green and blue image data) for every picture element, and the original image is reproduced on the basis of the separated color image data. In such an apparatus, inconsistency in color (color error or color difference) between the original image and the reproduced image frequently occurs when a color-reproducible range (gamut) of the image input medium is inconsistent with that of an image output medium such as a copying sheet, a color television or the like. That is, a color located within the color-reproducible range (gamut) of the image input medium is not completely reproduced, and in other words, the color is reproduced with insufficient color-reproducible when the color of the image input medium is out of the color-reproducible range (gamut) of the image output medium. Here, each of the image input media and the image output media is defined as a medium on which a visible image is formed or supported. The medium is referred to as an "image input medium" when the medium is used as an original from which an original image is read out by the image reading unit, while the medium is referred to as an "image output medium" when the medium is used as a recording medium to which the original image is recorded or reproduced. When the recording medium having an image thereon is used as an original, the recording medium is referred to as an "image input medium". Therefore, various techniques (color-correcting techniques) for correcting an input color image data have been utilized to correct such a color-inconsistency (decrease such a color error) and improve the color-reproducibility of the original image.
As one of the color-correcting techniques is well known a matrix-transforming technique as described in Japanese Unexamined Published Patent Application No. 63-92166. In this matrix-transforming process, a relationship between an original color data ( a digital density or lightness data) for an original image and a reproduced color data which is spectrometrically measured by observing an reproduced image under a predetermined observing environment (illumination), is beforehand obtained and approximately represented by a polynomial function for every picture element. A coefficient of the polynomial function for each picture element is stored in a matrix form (1). ##EQU1##
Here, R, G and B represent color image data (each representing density or lightness of each color component of each picture element) of the original image, aij (i, j=1, 2, 3) represents a color-correcting coefficient for the color image data, and C, M and Y represent color-corrected image data to be outputted. The above polynomial function and the matrix form are obtained for every picture element, and finally a color-corrected image data is formed using these polynomial functions and matrix forms.
However, the above color-correcting process has the following problem. If the recorded or reproduced color image thus color-corrected is observed under an observing environment different from the predetermined observing environment which is used to obtain the color-correcting matrix forms, the color of the recorded image is visually inconsistent with the color of the original image.
As another color-correcting technique is known a color compression technique (a data-compression transform technique). In the data-compression transform technique, an original color image data which is input from an image input medium such as a photographic film, a television, an ink-printed sheet or the like, is beforehand transformed into a recordable color image data which is compressed in an achromatic (neutral) color direction, and then a color image is recorded or reproduced on an image output medium such as a copy sheet, a television, a coated or non-coated sheet or the like on the basis of the compressed color image data.
The achromatic color is defined as a color having a lightness, but no hue and chromaticity (saturation). White, gray and black colors belong to the achromatic color, and are decreased in lightness in this order. In the following description, a color axis on which achromatic colors having various lightnesses (white, gray and black) are located is referred to as an "achromatic color axis". Further, the achromatic color direction is defined as a direction in which any color is directed to an achromatic color on the achromatic axis in a color space. As described below, the color-compression transforming process is a data-correcting process for shifting (color-compressing) a color within the color-reproducible range (gamut) of an image input medium to a color within the color-reproducible range (gamut) of an image output medium in the color space.
An image forming apparatus adopting the above color-compression technique includes an image reading system for reading out an original image from an image input medium such as a sheet, a photographic film, a color television or the like to form an original color image data, a color-compression transforming system for transforming the original color image data into the compressed color image data in an achromatic color direction, a matrix-transforming system for conducting a matrix-transforming process on the compressed color image signal to form a printer control signal which is used to sufficiently approximate a color of the reproduced color image to a color of the original image, and a recording system for recording or reproducing the color image from the printer control signal on an image output medium such as a copy sheet, a television or the like.
FIG. 1 shows color-reproducible ranges of various general image input and output medium such as a photographic color film, a television, an ink jet, a color print and so on in a chromaticity diagram of CIE-XYZ (CIE 1931 standard) colorimetric system. The color-reproducible ranges of the image input and output medium are defined as areas outlined by a solid line, a dotted line and so on in the chromaticity diagram as shown in FIG. 1. In the chromaticity diagram, an achromatic color is represented by a point C (x=0.33, y=0.33), and is located on the achromatic color axis. The achromatic color axis extends in a direction vertical to the X-Y plane of the chromaticity diagram, and has both ends representing white and black color. The achromatic color on the achromatic color axis is shifted up to the white color as a lightness thereof is increased, and down to the black color as the lightness is decreased.
As is apparent from FIG. 1, the color-reproducible ranges of the image input and output media are different in accordance with kinds of the image input and output media. Accordingly, in order to record or reproduce an original image whose color is within a color-reproducible range of the image input medium, but is out of a color-reproducible range (gamut) of the image output medium, it is necessary to correct a color image data of the original image such that the color of the original image is shifted to a suitable color within the color-reproducible range (gamut) of the image output medium in the chromaticity diagram (color space). In general, the original image data is corrected such that the color of the original image is shifted toward a point on the achromatic color axis and is located at a point within a color-reproducible of the image output medium. This color-correcting process is hereinafter referred to as "a color-compression process in an achromatic color direction". In the conventional image forming apparatus adopting the above color-compression process, a color-correcting data with which an input image data is color-corrected, is beforehand determined to an invariable data by a predetermined rule, and is beforehand stored in a memory of the apparatus.
Therefore, in the conventional image forming apparatus adopting the color-compression process, there frequently occurs that the data-compression process (color-correcting process) can not be sufficiently performed for some kinds of the image inputting systems because the invariable data is determined using a particular image inputting or output medium, and the reproduced color is inconsistent in hue or lightness with the original color when the original color is out of the color-reproducible range (gamut) of the image output medium (that is, the color-reproducible range gamut) of the image input medium is inconsistent with that of the image output medium).
Further, in the conventional image forming apparatus, even if the color-reproducible range (gamut) of the image input medium is consistent with that of the image output medium, the color-inconsistency between the original image and the reproduced image may occur when the reproduced image is observed under an observing environment such as illumination different from an observing environment for the original image.